Nb. I believe that Marcus has just left the UK for Australia so there may be a delay on responses.

From experience, services like Tweet2Download would fully automate the "authentication" step in the process, while you could combine user Form entry + a link checker PHP script to confirm that a user has added a link (e.g. "submit link location URL here"). Conversely you could hide information behind member-only walls on social platforms to incentivise engagement / sharing.

If automation isn't possible, you could always outsource the manual authentication for a small cost